The next scene displays the Morro Bay region in quasi-natural color, achieved using the IDRISI processing system by assigning band 1 to the blue band, band 2 to the green band, and band 3 to red band. (Note: this image, and others in the Tutorial, such as you saw in the Introduction, has a thin blue border; this means that you can click inside it and it will automatically enlarge to fill most of your screen. To close, press escape, then close, the the X button in the upper right.)
In practice, we use various color mapping algorithms to facilitate visual interpretation of an image, while analytical treatment usually works with the original DN (digital number) values of the pixels. The original DN values contain all of the information in the scene and though their range of values may make it necessary to remap them to create a good display, it doesn't add information. In fact, although visual interpretation is easier with the remapped image, remapping loses and distorts information thus, for analytical work, we use the original DN values or DN values translated to calibrated radiances.
With this mapping, twe see a pleasing and satisfying image because it depicts the world in the general color ranges with which we are naturally familiar. We can imagine how this scene would appear if we were flying over it at a high altitued.
We might comment that the ocean appears too dark in the IDRISI TM composite. This darkness is due to its general darkness in all of the bands, the consequence of water absorbing light from most of the visible range and, to a lesser degree, to the algorithm that IDRISI uses to combine three 8-bit bands into a single 8-bit composite image. The only exception is, of course, related to the breakers (here, with little internal structure) that are highly efficient at scattering light. The sediment patterns noted vary through bands 1 to 3 are clearly separable (rather than gradational) in this color rendition, with areas of greater load density having some green input (band 3 [red] shows only the densest sediment areas). Note especially the dark brownish-green tones by the delta where the coastal marsh develops, interpreted as a heavy load dumped at the mouth of the small river (p) flowing over it.
Probably the most unrealistic color in the rendering of the IDRISI version of this "true color" scene is the bright hill slopes. In this rendition, the base color is tan with some purplish overtones, but there are small areas and patches marked by a darker brown tone which, if inspected at ground level would have some green tints. The color composite shows these surfaces in a somewhat misleading way. From an airplane, we might describe these surfaces as a light golden-brown (from the dry grass, one reason California is often called the "Golden State"), similar to the terrain shown below which matches the first version's color mapping more closely.
The reason for the red-brown-purple color is that light from surfaces at certain orientations contribute considerable red from band 3, some blue (hence purple) from band 1, and little green from band 2.
The areas we presumed to be active grasslands (v) appear in "true color" as a slightly reddish brown. Areas correlated with fields and meadows are a medium brown. Most areas that were associated with medium to dark reds in the false color composite are very dark in the natural color version. This is particularly valid for the National Forest growth around (u), the stream-lining vegetation (i), and the wooded patches (l) scattered throughout the scene. The diminution of green color is explainable as follows: most of this vegetation is either California oaks, which have dark green leaves, or perhaps some evergreens which tend to be less reflective than deciduous plants, thus the dark brown color. These leaves, or needles, have generally lower reflectance in bands 1-3 but still are proficient reflectors (from cell walls) in band 4.
The coastal marsh (o) appears as a dark brownish-red in this true color rendering (as it seems to do in the aerial oblique photo). At least part of this effect may link to types of vegetation similar to that in the foreground of this photo, which we introduced earlier. It shows the edge of the marsh area well after the rainy season has started.
In this natural color version, the mysterious feature at s displays in its full glory, being a collage of bright green, arranged roughly in elongated strips. If you haven't discovered its identification already, we now reveal that it is an 18-hole golf course. We have extracted the portion of the digitized aerial oblique photo, that contains this tract, known as the Morro Bay Golf Course on State Park Road. This photo shows the lighter green fairways amidst darker green trees in close-up. The two bright patches within the image the might be sand traps. Their exagerated size in the photo is misleading, in that even if they are actually smaller than the 30 m resolution of a TM pixel containing one of them, the great brightness each contributes swamps the reflectance response of other classes in that pixel, making it appear overly light. This same phenomenon was observed early in the Landsat program where small [less than 8 m {26 ft} wide] dirt roads passing through sage brush or grasslands could be resolved because the pixels containing these pathways were substantially brightened.)
There are several parts of the image that contain distinctive orange-brown tones (as at x). This coloration is apparently due to soils that have a rich brown color. We also see this brown tone in the valley north and east of Los Osos and in part of that town. This valley appears to have its own small stream, which is not easily resolvable, or perhaps it once had a larger stream, which is now gone. Such a valley is probably underlain by alluvial deposits that typically are browner (enhanced by hydrous iron oxides similar to rust).
Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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